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Cite This: J. Am. Chem. Soc. 2019, 141, 11082−11092
Sustained Solar H2 Evolution from a Thiazolo[5,4‑d]thiazole-Bridged Covalent Organic Framework and Nickel-Thiolate Cluster in Water Bishnu P. Biswal,† Hugo A. Vignolo-Gonzaĺ ez,†,‡ Tanmay Banerjee,† Lars Grunenberg,†,‡ Gökcen Savasci,†,‡ Kerstin Gottschling,†,‡ Jürgen Nuss,† Christian Ochsenfeld,‡,§ and Bettina V. Lotsch*,†,‡,§,∥ †
Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany Department of Chemistry, University of Munich (LMU), Butenandtstraße 5-13, 81377 München, Germany § Center for Nanoscience, Schellingstraße 4, 80799 München, Germany ∥ Nanosystems Initiative Munich (NIM), Schellingstraße 4, 80799 München, Germany
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S Supporting Information *
ABSTRACT: Solar hydrogen (H2) evolution from water utilizing covalent organic frameworks (COFs) as heterogeneous photosensitizers has gathered significant momentum by virtue of the COFs’ predictive structural design, long-range ordering, tunable porosity, and excellent light-harvesting ability. However, most photocatalytic systems involve rare and expensive platinum as the co-catalyst for water reduction, which appears to be the bottleneck in the development of economical and environmentally benign solar H2 production systems. Herein, we report a simple, efficient, and low-cost allin-one photocatalytic H2 evolution system composed of a thiazolo[5,4-d]thiazole-linked COF (TpDTz) as the photoabsorber and an earth-abundant, noble-metal-free nickelthiolate hexameric cluster co-catalyst assembled in situ in water, together with triethanolamine (TEoA) as the sacrificial electron donor. The high crystallinity, porosity, photochemical stability, and light absorption ability of the TpDTz COF enables excellent long-term H2 production over 70 h with a maximum rate of 941 μmol h−1 g−1, turnover number TONNi > 103, and total projected TONNi > 443 until complete catalyst depletion. The high H2 evolution rate and TON, coupled with long-term photocatalytic operation of this hybrid system in water, surpass those of many previously known organic dyes, carbon nitride, and COF-sensitized photocatalytic H2O reduction systems. Furthermore, we gather unique insights into the reaction mechanism, enabled by a specifically designed continuous-flow system for non-invasive, direct H2 production rate monitoring, providing higher accuracy in quantification compared to the existing batch measurement methods. Overall, the results presented here open the door toward the rational design of robust and efficient earth-abundant COF−molecular co-catalyst hybrid systems for sustainable solar H2 production in water.
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chemical sensing,7 electronics,8 and catalysis.9 In spite of their versatility, there are only few reports on COFs utilized as photoabsorbers for photocatalytic H2 evolution so far.10 Although it is rare and expensive, all except one of these works has employed metallic platinum as the co-catalyst to reduce water efficiently, which appears to be the bottleneck in the development of scalable, economical solar H2 production. In addition, the use of nanoparticulate Pt co-catalysts precludes obtaining detailed insights into the nature of the catalytic sites and the intricacies of the photocatalytic cycle. Inspired by natural photosynthesis,11 researchers worldwide are motivated by this shortcoming to search for single-site, earth-abundant, non-precious metal-based co-catalysts with well-defined catalytic centers. So far, only one molecular co-catalyst−COF system for photocatalytic H2 evolution has been demonstrated.12 This system is based on an azine-linked COF (N2-
INTRODUCTION The conversion and storage of solar energy in the form of chemical bonds in “solar fuels” like H2 through light-driven water reduction has evolved into a key technology over the past decade due to the fast depletion of fossil energy sources and rapid global climate change.1 To drive the proton reduction half reaction in an efficient way, the major challenge is to find a catalytic system that is robust and highly active, but at the same time low-cost and earth-abundant, in combination with a strongly absorbing, chemically stable photosensitizer (PS).2 In this regard, covalent organic frameworks (COFs)3 have recently emerged as an exciting class of photoactive materials for light-driven H2 production due to their tunable light-harvesting4 and charge-transport properties.5 In contrast to other porous materials, COFs are known for being mechanically robust and offering large accessible surface areas. By virtue of their modular geometric and electronic structures, COFs have attracted significant interest for a range of applications including adsorption, storage and separation,6 © 2019 American Chemical Society
Received: March 25, 2019 Published: June 20, 2019 11082
DOI: 10.1021/jacs.9b03243 J. Am. Chem. Soc. 2019, 141, 11082−11092
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Figure 1. Synthesis and structural characterization of TpDTz COF. (a) Schematic representation of TpDTz COF synthesis. (b) Space-filling model of TpDTz COF pores with π−π stacking of successive 2D layers (gray, C; blue, N; red, O; yellow, S; and white, H). (c) Indexed PXRD patterns of TpDTz COF with corresponding Pawley refinement (red) showing good fit to the experimental data (blue) with minimal differences (cyan); the inset shows close-up of the indexed experimental (blue) and simulated (black) PXRD patterns based on Pawley fits [final Rwp = 2.59% and Rp = 1.89%].
latter enables efficient intermolecular π−π overlap that affords high electron and hole mobility.17 Such TzTz moieties further feature excellent photoabsorbing ability, which is likewise beneficial for photocatalysis.2a Nevertheless, TzTz-based COFs have not been explored so far. Notably, thus far, only a very limited number of COFs bearing photoactive functionalities such as triazine,10a,b diacetylene,10c or sulfone moieties10d have been shown to produce H2 from water, with the noble metal Pt acting as co-catalyst. Combining these aforementioned leverages, in this work, we present a light-driven hybrid proton reduction system employing a newly designed TzTz-linked COF (TpDTz) as a photoabsorber and a molecular Ni-thiolate cluster (NiME)18 assembled in situ from a Ni(II) salt and 2-mercaptoethanol (ME). The combination of the NiME cluster co-catalyst and TpDTz COF enables sustained H2 evolution with an excellent rate (941 μmol h−1 g−1) and a TONNi > 103 (70 h) in the presence of triethanolamine (TEoA) as the sacrificial electron donor (SED) in water under AM 1.5 light illumination. We thus report a single-site heterogeneous COF-based photocatalyst system that operates with a noble-metal-free cocatalyst in water as the solvent. We further carve out structure− property−activity relationships by comprehensively screening the parameter space of this heterogeneous−homogeneous hybrid photocatalytic system, including pH, SED, co-catalyst metal centers, different N/S-containing chelating ligands for
COF)10b acting as the PS and a cobaloxime molecular proton reduction catalyst, which shows a H2 evolution rate of 782 μmol h−1 g−1 and a turnover number TONCo = 54.4. However, the limited photostability and especially the utilization of an organic solvent (acetonitrile/water mixture; 4:1) were major concerns.12 Notably, a majority of molecular catalysts decompose during prolonged catalysis, are inherently insoluble in water, and require the addition of organic solvents to accomplish water reduction.13 With cobaloxime-based systems, for example, the catalyst often converts to an inactive form within a few hours ( 103 (70 h) and a TOF = 2.3 h−1 when the system is fully active were obtained. A mathematically projected (Supporting Information section S8) TONNi > 443 (890 μmol of total H2 evolution) can be obtained for the photocatalytic H2 evolution performance corresponding to a complete depletion of the co-catalyst. The relation between co-catalyst, SED, and observed activity loss of the system in time was confirmed by in situ addition of loss-equivalent amounts of ME or TEoA independently after 72 h of illumination, which did not change the deactivation trends observed (Supporting Information, section S8). It must be noted that the TONNi mentioned above is only a lower limit calculated based on the total amount of Ni(II) salt used for the photocatalysis experiment. Under identical conditions the erythrosin B (EB) dye-sensitized system18 produces H2 with a maximum rate of 49 297 μmol g−1 h−1 (attained in 1 h); however, the rate rapidly drops off, and the whole system becomes completely inactive within 7 h. A TONNi > 36.5 (73 μmol of total H2 evolution) was obtained after 7 h (TOFNi =
organic dye PSs, which hinders the long-term performance of the photocatalytic system.15a,18 The strategy of combining a photochemically stable COF photoabsorber with the Nithiolate cluster co-catalyst in water could thus be a viable path to impart better long-term stability for H2 production. The beauty of the aforesaid NiME complex lies in its simple, quick in situ synthesis in water upon addition of a Ni(II) salt and ME at room temperature. This in situ assembly strategy is different from those of most other Ni(II) and Co(II) cocatalyst complexes, featuring arduous ex situ synthesis and purification of a water-soluble analogue, thus adding to the cost-effectiveness of the NiME cluster co-catalyst approach.12,15 In addition, this cluster has been shown to be a potent H2 evolution co-catalyst producing H2 immediately after light illumination in the presence of a PS and SED, and hence does not require any photodeposition, nor does it show an activation time, contrary to Pt-based photocatalytic systems.10 For better measurement accuracy and to gain insights into the photocatalytic mechanism, we developed a continuous-flow system to monitor the H2 evolution performance of the hybrid photocatalytic system (Figure 3 and Figure S23). In this measurement system, the molar flow entering the system, the pressure (0.5 bar), and the temperature (25 °C) of the reactor are continuously controlled, while bypassing some of the outflow to an open sampling loop gas chromatograph (GC) autosampler. In this way, the rate of H2 production (RH2) can be monitored directly using only two experimental inputs: Fin from the mass flow controller, and xH2,ppm from an online GC (BID) detection system (eq 1), where Fin is the carrier gas (in this case He) flow in to the system and xH2,ppm is the molar fraction of H2 at the outlet. R H2 =
Fin × x H2,ppm × 10−6 (1 − x H2,ppm × 10−6)
(1)
Typically, the run-to-run error with this method is below 3%, compared to at least 15−20% error with a standard batch system. Also, this continuous-flow system is independent of 11086
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Figure 4. Photocatalytic H2 evolution. (a) Comparison of photocatalytic H2 evolution rates in water (H2O) and deuterium oxide (D2O), using TpDTz COF over 72 h and EB dye under AM 1.5 light irradiation [COF photosensitizer: 5 mg of TpDTz COF in 10 mL of H2O/D2O with 10 vol % TEoA, 0.5 mg of Ni(OAc)2, and 1.4 μL of ME at a final pH of 8.5; dye photosensitizer: 1.33 mg of EB in 10 mL of H2O with 10 vol% TEoA, 0.5 mg of Ni(OAc)2, and 1.4 μL of ME at a final pH of 8.5]. (b) Light on−of f cycles for photocatalytic H2 evolution experiments with TpDTz COF in water over 26 h. (c) Photocatalytic H2 evolution with TpDTz COF in water using different co-catalysts. (d) Photocatalytic H2 evolution with TpDTz COF in water using different metal-ME co-catalysts. (e) Photocatalytic H2 evolution from water using different photosensitizers. (f) Overlay of the UV−vis DR spectra of TpDTz COF with apparent quantum efficiency (AQEs) for the photocatalytic H2 evolution reaction with TpDTz COF at four different incident light wavelengths.
31.9 h−1), which is 12 times lower than the value projected for the TpDTz COF-sensitized system (Figure 4a). These results demonstrate the added value of using a heterogeneous PS to stabilize charge transfer in photocatalytic hybrid systems. Also, the TpDTz COF-NiME photocatalytic system produces H2 at a 17% higher maximum rate and has a TON nearly 8-fold as high as our previously reported N2-COFcobaloxime-based system (782 μmol h−1 g−1, TONCo = 54.4), while operating in water.12 The H2 evolution rate and the
sustained activity of this simple TpDTz COF-NiME system are competitive with and even superior to those of many COFbased photocatalytic systems (Table S3) and other benchmark photocatalytic systems involving metallic Pt or molecular Ni co-catalysts. Examples include g-C3N4/Pt (840 μmol g−1 h−1),25b TP-BDDA/Pt (324 μmol g−1 h−1),10c N2-COF/Pt (480 μmol g−1 h−1),10b crystalline poly(triazine imide)/Pt (864 μmol g−1 h−1),25a sg-CN-Ni (103 μmol g−1 h−1),25c Ni12P5/g-C3N4(536 μmol g−1 h−1),25d NCNCNx-NiP (763 μmol 11087
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Journal of the American Chemical Society g−1 h−1),25e and carbon quantum dots (CQDs)-Ni (398 μmol g−1 h−1).25f Control experiments were performed by sequentially removing one of the components, i.e., TpDTz COF, TEoA, Ni(OAc)2·4H2O, and ME, at a time from our photocatalytic system to identify their importance and role for the H2 evolution. Indeed, no H2 evolution was observed for a period of 12 h unless all individual components act in concert, signifying that each is essential for the photocatalytic system to work and efficiently produce H2 (Figure S27). Furthermore, a 1:10 metal-to-ligand molar ratio and 10 wt% of catalyst with respect to the PS were observed to elicit the best photocatalytic performance (Figures S28 and S29). To confirm water as the source of H2, the photocatalytic reaction was performed in D2O under identical conditions (Figure 4a). A production rate for D2 in D2O similar to that for H2 in H2O was observed over 72 h, taking batch-to-batch variations into account. This result suggests that water is the hydrogen source responsible for the production of H2, assuming that no significant proton/deuterium exchange processes in the individual components are at play. This finding was further confirmed by an almost complete disappearance of the m/z = 2 signal for H2 in a mass spectrometric measurement of the headspace gas of the photocatalytic reaction performed in D2O (Figure S26). Note that D2 is evolved with a time lag compared to H2, which is likely due to the kinetic isotope effect (KIE) of deuterium as described below. Further, H2 evolution experiments performed under multiple light on−of f cycles over a period of 26 h (Figure 4b) suggests a purely light driven H2 evolution process in water. Once the catalytic system is fully active, H2 evolution activity is seen to be restored even after a prolonged light off period. SED and the reaction pH are known to have a profound influence on the activity of many H2 production systems.10b,c,12 In our case, we observed a similar effect; the rate of H2 generated from the photochemical reaction is the highest (941 μmol g−1 h−1) at pH 8.5 using TEoA as SED. However, at acidic conditions (pH 6.5) there was negligible H2 evolution (16 μmol g−1 h−1). This could be attributed to the protonation of TEoA or due to inhibition of proton loss from one-electron oxidized TEoA+.13c Notable H2 evolution is observed over 24 h under alkaline conditions (pH 11), albeit at lower rates (308 μmol g−1 h−1) as compared to pH 8.5 (Figure S30). This is possibly due to the reduced driving force for protonation of the Ni hydride intermediate co-catalyst species at higher pH to subsequently generate H2. Triethylamine (TEA) and Na2S were also explored as potential SEDs. Interestingly, they produce H2 but with significantly lower rates of 84 μmol g−1 h−1 and 7 μmol g−1 h−1, respectively (Figure S31). Higher TEoA concentrations were found to decrease H2 evolution rates; a TEoA concentration of 10 vol% in water was observed to result in the maximum H2 production rate (Figure S32). H2 evolution rates of the photocatalytic systems containing TpDTz COF PS and different Ni(II)co-catalysts were measured (Figure 4c). Different sulfur-containing compounds, such as thiourea (TU) and 2-mercaptophenol (MP), were explored as potential ligands for in situ formation of Ni(II)cocatalyst complexes. However, neither NiTU nor NiMP produced any H2 with TpDTz COF, possibly due to unfavorable complexation of the ligands with Ni(II) in water: TU and MP are known to be poorer complexation agents as compared to ME.26 Also, a reported ex situ synthesized Ni(abt)2 complex15c was studied as a potential
H2 evolution co-catalyst under our experimental conditions, but no H2 evolution was seen, most likely due to its poor solubility in water. It is also interesting to note that TpDTz COF produces H2 with a significantly smaller rate of 23 μmol g−1 h−1 with metallic Pt co-catalyst and TEoA at pH 8.5 over a period of 24 h as compared to that with NiME. The significant difference between H2 evolution of the molecular co-catalyst and photodeposited Pt nanoparticles is difficult to explain by a single effect.12,25f However, it may be argued that the higher activity of the NiME co-catalyzed system in contrast to the surface bound Pt nanoparticles (Figure S48) is due to a more effective blocking of charge carrier recombination since the cocatalyst is physically separated from the framework (physisorbed), which may support better charge separation. We further screened the H2 evolution activity of other transition metal-ME complexes, such as CoME and CuME, with TpDTz COF as the PS following a similar method as that of NiME. Although all systems produced H2, they do so with a much lower rate following the order NiME (941 μmol g−1 h−1) > CoME (85 μmol g−1 h−1) > CuME (52 μmol g−1 h−1). This could be due to the poor solubility of CoME and CuME clusters in water compared to the NiME, which is in accordance with the reported dye sensitized molecular system18 (Figure 4d). We then evaluated the H2 evolution ability of the NiME cluster co-catalyst with a variety of photoabsorbing materials; TpDTP COF,19 N3-COF,10b an amorphous porous polymer containing TzTz groups (TzTz-POP-3),27 and the diamine linker DTz were tested under identical conditions. Even though N3-COF is considered one of the most active COFs for photocatalytic H2 generation (reported rate of 1700 μmol g−1 h−1 when co-catalyzed by Pt),10b with NiME co-catalyst it produces H2 only at a very low rate of 40 μmol g−1 h−1 (Figure 4e). Under similar conditions TpDTP COF produces H2 at a rate of 160 μmol g−1 h−1, which is nearly 6 times less compared to that of the TpDTz COF sensitized system. The marked difference in photocatalytic activity between TpDTz COF and TpDTP COF may in part be rationalized by the reaction conditions which were not optimized specifically for TpDTP COF, but also by their different photon absorption characteristics. A redshift of ∼67 nm is observed for TpDTz COF with respect to TpDTP COF, which indicates that TpDTz COF absorbs photons more effectively in the visible range. This said, increased reactivity is only expected if the conduction band is not significantly lowered to maintain the thermodynamic driving force for the HER. In addition to that, the higher crystallinity and the higher BET surface area of 1356 m2 g−1 for TpDTz COF versus 736 m2 g−1 for TpDTP COF, along with a better dispersibility of the more hydrophilic TpDTz COF in aqueous solution, are likely determining factors for the enhanced photocatalytic activity. Notably, the amorphous polymer TzTz-POP-3 and the diamine linker DTz are completely inactive at producing H2 with the NiME cocatalyst (Figure 4e). Overall, the significantly lower reactivity of other PSs in producing H2 with the NiME co-catalyst is rationalized by a combined effect of unfavorable chargetransfer processes, reduced light harvesting, low crystallinity and surface area, and poor dispersibility in water. The apparent quantum efficiency (AQE) was calculated using four different bandpass filters with central wavelengths (±20 nm) at 400, 500, 550, and 600 nm to quantify the spectral contribution toward H2 evolution activity of the TpDTz COF photoabsorber. Figure 4f shows that TpDTz 11088
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Figure 5. Reaction limitations insights. (a) General schematic of the proposed pathway for H2 evolution (color code: gray, C; red, O; yellow, S; blue, N; and light pink/white, H). (b) Proposed key steps of the photocatalytic H2 evolution reaction with TpDTz COF and NiME cluster cocatalyst. [Ni-L] denotes a ligand-coordinated co-catalyst state which is attained fast compared to the [R] state, [R] denotes the catalyst resting state, which is catalytically active nickel cluster species, [D] denotes the deactivated species, and [I] denotes an intermediate reduced catalyst species able to run the HER step.
reach maximum rates in the first run (Figure 4a), the absence of this activation time during light on−off cycles or long dark periods before illumination (Figure 4b), and the KIE in D2O, namely, smaller deuterium evolution rates and delayed response, together with the observation of a similar initial activation time as with H2O (Figure 4a). A possible reaction model is outlined in Figure 5b. The absence of an activation time during light on−off cycles (Figure 4b) suggests a light enhanced formation of a catalyst resting state [R] of the NiME complex upon illumination, as seen by the initial activation time required for the system to reach maximum efficiency. The reaction network can then be reduced to the following core steps using a microkinetics analysis. For the heterogeneous (COF) fast cycle: COF photoexcitation (hν), exciton recombination in the COF (krec), reductive quenching of the COF (kq), and electron transfer from COF•− to [R] to form the active intermediate species [I] (ka). Quantum-chemical calculations on the TpDTz pore model system (Figure S55) identify the lowest photoexcitation energy to be 2.30 eV (Table S9), the difference density of this excited state (Figure S57) visualizing the exciton. The spin density of the radical anion as a result of the reductive quenching of this state is shown in Figure S58. For the homogeneous (catalyst) cycle: formation of a rapidly coordinated complex [Ni-L] (Keq), slow assembly of the catalytically active species (kT, kT−1), an apparent first-order activation step from [R] to [I] (ka), an irreversible deactivation step [D] (kd), and the closing of the catalytic cycle via a dark step that produces H2 (kHER). If in such a system kT and kT−1 are significantly slower compared to the rest of the steps, the on−of f behavior can be explained, because in the absence of light the dark equilibrium is slow and the amount of [R] and [I] will not change significantly over time. Furthermore, the absence of HER in the dark during the light on−of f cycles observed in Figure 4b suggests that a kHERlimited homogeneous cycle is unlikely. Then, once nickel enters the cycle as [R] more rapidly due to a light-shifted equilibrium, [R] will build up until the rate of [D] leaving the system irreversibly is equal to the rate of formation of [R], [I] being stationary. This will lead to an expression for activation
COF has a maximum AQE of 0.2% at 400 nm. Under AM 1.5 illumination, the AQE was estimated to be 0.044%, which is higher than that of our previously reported N2-COFcobaloxime H2 evolution system (0.027%).12 We further verified the photochemical stability of TpDTz COF after a 72 h long photocatalysis experiment. The isolated TpDTz COF sample was fully characterized using PXRD, ssNMR, SEM, and TEM, and it was found that the framework structure, crystallinity, and morphology of TpDTz COF are retained (Supporting Information, section S9), thus supporting the high chemical stability of this COF (vide supra). A small additional signal at 56.6 ppm in the 13C ssNMR spectrum possibly corresponds to trapped ME molecules inside the TpDTz COF pore. This, together with the observation of traces of Ni in the post-photocatalytic TpDTz COF using SEM-EDAX (Figure S46) may hint to chemisorption of small amounts of co-catalyst to the COF walls. However, 15N ssNMR of the TpDTz COF sample does not show any noteworthy difference in the chemical shifts of the N signals before and after photocatalysis (Figure S41), suggesting that there is no substantial direct interaction between the residual Ni and the nitrogen centers of the COF. Also, the as-recovered TpDTz COF sample does not produce any H2 under identical photocatalytic conditions except for the absence of Ni(OAc)2· 4H2O and 2-ME ligand. This suggests that the interaction between TpDTz COF and NiME is mostly physical, and no lasting chemical interaction exists between the two components. Our finding thus suggests an outer-sphere electron transfer to be at play, which nevertheless is efficient enough to allow facile charge transfer from the photoabsorber to the NiME co-catalyst (Figure 5a). To obtain deeper insights into the photocatalytic mechanism, an overall coarse-grain mathematical model (Supporting Information eq S6.2) of the photocatalytic reaction was developed (see Supporting Information section S8 for details) by taking advantage of the quantification of hydrogen evolution rates in our flow detection platform. Our model was based on three primary experimentally observed trends: the activation time required by the photocatalytic system to 11089
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Journal of the American Chemical Society of the photocatalytic system such that the activation curve is an exponential asymptote with time constant ta, which is dominated by electron-transfer rate ka, a linear deactivation with an apparent time constant td, an HER apparent kinetic constant RH2,max, and the apparent transient time t0 only which corresponds to the initial time delay necessary for the intermediate [I] to be pseudo-stationary (Supporting Information eq S6.2). Our model further provides an accurate fit to the data obtained in both H2O and D2O, in line with an expected trend of a KIE. In this case, RH2,max, td, and t0 changed as expected, but the time constant for the activation step (ta) being independent of the isotopic mass is almost unchanged (Table S2), further corroborating our proposed rate-limiting steps (RLS). Our reaction model not only explains these qualitative trends but also provides an accurate fit with standard errors below 5% for different data sets. It is important to note that acquiring detailed insights into the H2/D2 evolution reaction mechanism for the TpDTz COF-NiME photocatalytic system became possible only with the use of a flow reactor system. Our reaction modeling results suggest that as long as a slow catalyst activation time is observed, the RLS of the system is seemingly the electron transfer from the COF to the NiME complex. While this outcome is fully consistent with the assumed outer-sphere electron-transfer process, it reinforces the idea of studying the kinetics of such processes in more detail as this will be crucial to improve the HER rate by rational design of the COF−co-catalyst interface.
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AUTHOR INFORMATION
Corresponding Author
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[email protected] ORCID
Bishnu P. Biswal: 0000-0002-8565-4550 Tanmay Banerjee: 0000-0002-4548-2117 Gökcen Savasci: 0000-0002-6183-7715 Jürgen Nuss: 0000-0002-0679-0184 Christian Ochsenfeld: 0000-0002-4189-6558 Bettina V. Lotsch: 0000-0002-3094-303X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS B.P.B. acknowledges the Alexander von Humboldt foundation for a research fellowship. B.V.L. acknowledges financial support by an ERC Starting Grant (project COF Leaf, grant number 639233), the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - project number 358283783 - SFB 1333, and the Max Planck Society. B.V.L. and C.O. acknowledge further support by the cluster of excellence econversion (EXC 2089) and the Center for Nanoscience (CeNS). C.O. acknowledges financial support as a MaxPlanck-Fellow at the Max Planck Institute for Solid State Research, Stuttgart. We thank Viola Duppel for recording the SEM and TEM images, Igor Moudrakovski for solid-state NMR measurements, Marie-Luise Schreiber for elemental analysis, and Prof. Dr. Ulrich Starke and Dr. Kathrin Müller for XPS data.
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CONCLUSIONS We report the first COF photosensitizer and noble-metal-free molecular co-catalyst photocatalytic system for sustained solar H2 production from water. This single−site system comprises the newly designed N/S-containing TpDTz COF PS that absorbs strongly in the visible region of the solar spectrum and is robust for long-term hydrogen evolution. In combination with an earth-abundant Ni-thiolate cluster co-catalyst which self-assembles in water, we obtain solar H2 evolution rates as high as 941 μmol h−1 g−1 and a TONNi > 103 (70 h) with persistent H2 evolution for more than 70 h in a single run, which surpasses many benchmark photocatalytic H2 evolution systems based on COFs and carbon nitrides. To map out the parameter space of this hybrid photocatalytic system, we comprehensively screened the influence of various reaction components, including pH, SED, co-catalyst metal centers, different N/S-containing chelating ligands, and a variety of PSs on the photocatalytic activity. In addition, we have introduced a newly designed continuous-flow system enabling the noninvasive, direct detection of the H2 production rate. This platform not only provides higher accuracy in quantification; it also paves the way for unprecedented insights into the reaction mechanism which are difficult to obtain with the existing batch measurement methods. Microkinetic modeling of the reaction system suggests that an outer-sphere electron transfer from the photoabsorber to the catalyst is the rate-limiting step, thus spotlighting the importance of the rational design of the COF−co-catalyst interface.
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Synthesis, crystallography, characterizations, and photocatalysis experimental details, including Figures S1−S58 and Tables S1−S9 (PDF)
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REFERENCES
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DOI: 10.1021/jacs.9b03243 J. Am. Chem. Soc. 2019, 141, 11082−11092
Article
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